In natural photosynthesis, the energy of absorbed sunlight produces energized carriers that execute chemical reactions in separate regions of the chloroplast. The electrons used to produce nicotinamide adenine dinucleotide phosphate (NADPH) are excited via the “Z-scheme” of light-absorbing photosystems I and II. The energy of the photoexcited charge carriers is then used to overcome the thermodynamic barrier and to provide any kinetic overpotential needed to drive the photosynthetic reactions.
Compared to the excitation of a single light absorber, excitation of the two light absorbers, or a “Z-scheme” system, allows capture of lower energy photons and thus a larger part of the solar spectrum, which can potentially lead to a higher efficiency. Moreover, photosystems I and II are arranged side by side on the thylakoid membrane with the electron transport chain between them for efficient charge transfer. In addition, the spatial separation of the reduction and oxidation catalytic centers minimizes the undesirable back-reaction of the photosynthetic products. This careful arrangement of photosynthetic constituents results in a fully integrated system that facilitates conversion of solar energy into chemical fuels. The average rate of energy captured by this photosynthetic process approaches 130 terawatts, about six times larger than the current worldwide power consumption.